As semiconductor technology has constantly been improving, the use of field effect transistors (FETs) has become more prevalent in all facets of computer and communications technology. This technological improvement has allowed for faster operation and more compact arrangement of FETs within integrated circuit (IC) chips. IC chips are widely used in all electronic equipment, including equipment that is manufactured and operated in harsh environments. However, such harsh environments increase the likelihood of exposure of IC chips to high-voltage electrostatic discharge (ESD) events, to which IC chips are vulnerable. The high current that results from ESD events breaks down the internal semiconductor material of the FETs, resulting in damage to the IC chip. The vulnerability of IC chips to ESD events has created an important need for ESD protection circuits.
Typically, ESD circuits are designed to protect a given device from ESD events that occur from a given terminal or node to ground. However, in some applications, a ground potential can experience a voltage shift or a pin can have a negative voltage transient relative to a substrate, which could be grounded. For example, in an automotive environment, ground can shift approximately 2 volts across the body of a car. Such negative voltage events can result in significant leakage current from ground to a given terminal through the ESD protection device, which may result in damage to the ESD protection device.
One solution to this and other negative voltage shifts is the implementation of ESD protection devices in series with each other, each providing a respective direction of ESD protection. However, providing series connected ESD protection devices may not be able to provide the desired voltage characteristics for ESD protection in both directions. For example, the ESD current may not flow uniformly through the device, such that one of the ESD protection devices may break down during an ESD event in one or both directions. In addition, such an arrangement can be both cumbersome and costly. Another potential solution can include a resistor connected in series with the ESD protection device. However, such an arrangement can create an unacceptable voltage drop in the typical operation of the IC and can potentially degrade the ESD protection capability.